The rounded top of the board bore a painted 32-point compass pattern. Each point featured a line of eight holes radiating from the center of a circle. The lower, square portion of the board had horizontal lines of holes under columns that represented the speed of the ship in knots.
During each standard four-hour watch, the crew measured the ship’s speed and direction eight times, every half hour, and recorded them using pegs: direction under the appropriate compass point on the rounded top; speed along the bottom. After each watch, the navigator collected the data, logged it, plotted it on a chart, cleared the board, and then began the process again.
The boards were widely used throughout Europe and Scandinavia from the late 15th century until the mid-19th century.More information at Hakai Magazine.
Photo credit: Gjalt Kemp Scheepsantiek/ships-antiques.com